1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine.
2. Description of the Related Art
One known internal combustion engine such as a vehicular engine is provided with an EGR mechanism which returns some of the exhaust gas that flows through an exhaust passage to an intake passage in order to reduce the amount of nitrogen oxides (NOx) in the exhaust gas. This EGR mechanism includes an EGR passage through which some of the exhaust gas in the exhaust passage flows into the intake passage, and an EGR valve which is driven open and closed to change the amount of exhaust gas that flows from the EGR passage into the intake passage (in this specification, this exhaust gas will also simply be referred to as “EGR gas”). When some of the exhaust gas of the internal combustion engine is returned to the intake passage using the EGR mechanism in this way, it results is there being some gas which does not contribute to combustion (i.e., the exhaust gas) in the combustion chamber of the internal combustion engine when fuel is burned in the combustion chamber. As a result, the combustion temperature of the fuel in the combustion chamber decreases, which reduces the amount of NOx that is produced, such that the exhaust gas of the internal combustion engine contains less NOx.
Also, in an internal combustion engine provided with an EGR mechanism, an abnormality determining process for determining whether there is an abnormality in the mechanism opens and closes the EGR valve and determines whether there is an abnormality based on a change in the intake pressure of the internal combustion engine at that time. However, execution of this abnormality determining process requires an engine operating state in which the intake pressure of the internal combustion engine is stable, and execution of the abnormality determining process may be difficult in an internal combustion engine in which the EGR valve is closed when the engine is in such an operating state. This is because when the abnormality determining process is executed while the engine is operating with a stable intake pressure, the EGR valve which is closed opens, and as it does so, exhaust gas is recirculated (i.e., EGR is performed) to the intake passage, which is not done during normal engine operation and may cause the internal combustion engine to stall or the like.
Therefore, as described in Japanese Patent Application Publication No. 2002-4901 (JP-A-2002-4901) (paragraphs [0002], [0030], and [0031]), it is conceivable to execute the abnormality determining process described above during a fuel cut in the internal combustion engine, in which case the engine is in a state in which the intake pressure is stable and engine operation is not affected by EGR. Incidentally, a fuel cut in the internal combustion engine is performed when i) the vehicle provided with the engine is decelerating, ii) the accelerator is not being depressed, and iii) the vehicle speed is equal to or greater than a predetermined value greater than 0. In JP-A-2002-4901, the abnormality determining process temporarily opens the EGR valve, which is closed, during a fuel cut, and determines whether there is an abnormality in the EGR mechanism based on a change in the intake pressure at that time. As a result, the abnormality determining process is executed without causing engine stall or the like, and it can be determined whether there is an abnormality in the EGR mechanism through this process. Incidentally, it is presumed that after determining whether there is an abnormality in the EGR mechanism during a fuel cut, the EGR valve, which is temporarily open, will be closed.
It is also conceivable that the fuel cut may be stopped from the vehicle coming to a sudden stop or accelerating or the like while the EGR valve is temporarily open from execution of the abnormality determining process during the fuel cut before whether it has been determined there is an abnormality of the EGR mechanism. In this case, the abnormality determining process is cancelled, and the EGR valve, which has been temporarily opened, is closed, which is the state of the EGR valve prior to execution of the abnormality determining process, i.e., the normal state of the EGR valve. Also, fuel injection in the internal combustion engine resumes when the fuel cut stops, and the fuel injection quantity of the engine is a value which corresponds to the engine operating state, e.g., the engine speed and engine load and the like, at that time. More specifically, the fuel injection quantity at this time is a value that enables the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine to be an appropriate value on the condition that the EGR valve is closed and EGR is not being performed.
However, when the EGR valve is temporarily open from executing the abnormality determining process during a fuel cut, the following inevitably occurs in connection to the fact that the EGR passage is filled with air, when the abnormality determining process is cancelled when the fuel cut stops.
That is, even if the EGR valve, which is temporarily open from executing the abnormality determining process, is closed when that process is cancelled, air in the EGR passage (i.e., the EGR mechanism) flows into the intake passage at that time, causing the amount of air in the combustion chamber to become excessive by that amount with respect to the fuel injection quantity after fuel injection resumes in the internal combustion engine. Incidentally, the reason why air in the EGR passage flows into the intake passage when closing the EGR valve when the abnormality determining process is cancelled is because there is a response delay both in the closing the EGR valve and in the reduction of the flowrate of gas (i.e., air) that flows through the EGR passage and into the intake passage when the EGR valve is closed.
Therefore, after the abnormality determining process is cancelled when the fuel cut stops and fuel injection in the internal combustion engine resumes, the air-fuel ratio of the air-fuel mixture in the combustion chamber inevitably becomes leaner than the appropriate value until the flowrate of the gas that flows from the EGR passage into the intake passage finishes decreasing when the EGR valve closes. When the air-fuel ratio of the air-fuel mixture in the combustion chamber becomes leaner than the appropriate value in this way, combustion of the air-fuel mixture may become unstable, possibly resulting in a decrease in engine speed or engine stall.